The present invention relates to a bearing part material for forming bearing parts such as a race of a rolling bearing assembly.
The life of a rolling bearing assembly is highly dependent upon wear on a raceway surface or occurrence of surface flaking such as called pitting. Accordingly, a bearing part constituting a bearing of the rolling bearing assembly or the like employs a material such as bearing steel of high carbon content or carburizing steel capable of being formed with a carburized layer at its surface area through a carburizing treatment. Further, the material for bearing parts is subjected to a heat treatment such as a hardening/tempering treatment for transforming at least its surface layer constituting the raceway surface to a martensitic structure, and thereby a required hardness for preventing wear as well as a required toughness for preventing pitting are ensured.
More recently, however, bearing assemblies for use in automobiles have become unable to satisfy the life requirements because conditions under which they operate have become more severe. This has brought about a demand for a material for bearing parts which is capable of satisfying the life requirements for such bearing assemblies. Hence, it has become the practice in the art to apply a shot peening treatment to the heat treated material for bearing parts thereby imparting compressive residual stress to the raceway surface for the purpose of improving the life of the bearing assemblies.
Unfortunately, even when the shot peening treatment is applied, the bearing assemblies may sometimes encounter variations in the life. As an approach to ensure that the life requirements are satisfied, a shot time during the shot peening treatment is increased. However, this approach involves a problem that the bearing assemblies are produced at lower efficiency and hence, at higher costs.
In view of the foregoing problem of the prior art, the present invention has an object to provide a material for bearing parts which is capable of satisfying the life requirements for bearing assemblies without an excessive increase in the shot time during the shot peening treatment.
In accordance with the present invention for achieving the above object, a material for bearing parts comprises a steel hardened by a heat treatment and having a tempered martensitic structure at its surface layer to be subjected to a shot peening treatment, wherein the tempered martensitic structure contains carbide with a mean particle size of less than 1 xcexcm and an area percentage of 10-15%, and residual austenite in an amount of 7-14%, and has a hardness of HRC=61-65.
In the material for bearing parts of the above composition, plastic deformation can be produced as extended from the surface toward deep inside without increasing the shot time during the shot peening treatment. This permits the residual stress to be effectively imparted to the material for bearing parts, so that the material may form bearings intended for use under severe conditions which feature an increased longevity and smaller life variations, and thereby the material can satisfy the life requirements. In addition, it is possible to offer the bearing part material for producing bearings at high efficiency and low costs because the material negates the need for excessively increasing the shot time during the shot peening treatment.
Specifically, the present inventors have found that the varied life of bearings constructed from the conventional bearing part material derives from the variations in quality of the material which has been heat treated and is yet to be subjected to the shot peening treatment. The present inventors also found that the heat treated bearing part material may be properly adjusted for the mean particle size and area percentage of carbide contained therein, for the amount of residual austenite contained therein and for the hardness thereof and then be subjected to the shot peening treatment for imparting thereto the plastic deformation extended from the surface toward deep inside whereby the utilization of the effect of the compressive residual stress is maximized for achieving the life improvement. Based on such findings, the present invention has been accomplished.
The reason for defining the mean particle size of carbide as less than 1 xcexcm is because the presence of carbide 1 xcexcm or more in size limits the compressive residual stress imparted by the shot peening treatment within a portion of a shallow depth from the surface.
The reason for defining the area percentage of carbide as 10-15% is because with the area percentage thereof in excess of 15%, the compressive residual stress imparted by the shot peening treatment is limited within the portion of the shallow depth from the surface whereas with the area percentage of less than 10%, rolling contact fatigue life is reduced.
The reason for defining the amount of residual austenite as 7-14% is because if the residual austenite is present in an amount of less than 7%, the transformation of the residual austenite cannot provide sufficient compressive residual stress; on the other hand, if the residual austenite is present in an amount of more than 14%, residual austenite in a surface layer is referentially transformed to martensite, inhibiting the transformation of residual austenite in an inside portion, so that the compressive residual stress cannot be imparted so far as to the inside.
The reason for defining the hardness as HRC=61-65 is because with the hardness in excess of 65, the shot peening treatment produces insufficient plastic deformation, thus failing to impart the compressive residual stress so far as to the inside; with the hardness of less than 61, on the other hand, the shot peening treatment produces too much elastic deformation which means reduction of the plastic deformation and hence, the compressive residual stress cannot be imparted so far as to the inside just as the foregoing.
As a steel employed as the above bearing part material, a bearing steel is preferred.
In this case, the material may be imparted with the compressive residual stress to an extent of an even greater depth from surface, resulting in more effective life improvement. Thus is attained a more preferable material for bearing parts.